You should review completed (and failed - but that is different) changes, to determine how effectively they were implemented, whether they highlighted any weaknesses in your procedures, whether they were completed on time and within budget and if not why not, whether they have achieved the expected results, whether they caused any particular difficulties, whether they highlighted any scope for improvement, etc. etc.

Sending notification to the customer or the requestor does not of itself close the change._________________"Method goes far to prevent trouble in business: for it makes the task easy, hinders confusion, saves abundance of time, and instructs those that have business depending, both what to do and what to hope."
William Penn 1644-1718